DIGESTION OF STARCH 267 



condensed and insoluble forms for storage. All such substances 

 must undergo digestion before leaving the cell in which they are 

 deposited. When a stream of material sets in from a place of 

 storage, or formation, to a tissue using or restoring the translo- 

 cated food, it often occurs that the material is not used so rapidly 

 as it is moved, in which case it is converted into insoluble forms 

 and stored in transit in convenient cells. Before such substances 

 stored by the wayside can be moved farther they must undergo 

 digestion again. An instance of this is afforded by starch formed 

 in mesophyl cells in leaves. It undergoes digestion into maltose 

 before leaving the cell, and may be converted back into starch in 

 the next, and so on numberless times before finally being assimi- 

 lated, and yielding its energy to living matter. 



Simple organisms like bacteria and fungi cany on extra-cellu- 

 lar digestion by excreting enzymes which act upon the substances 

 in the medium in which they live, and the results of the digestion 

 are absorbed. Parasitic forms use an enzyme to dissolve the 

 walls of the host and allow them to penetrate the protoplasts, 

 and the extending tubes of pollen cells are provided with a simi- 

 lar means of boring down through the style of the flower. 



Extra-cellular digestion is also effected by species of the Ne- 

 penthes family in which a proteolytic enzyme is excreted by the 

 glandular cells lining the pitchers, and the fluid contained in the 

 pitchers is thus enabled to digest the bodies of animals entrapped. 

 Dionaea, Drosera, and other carnivorous species show a similar 

 adaptation. 



The embryos of a large number of species are enclosed with 

 an endosperm containing stored food, which is digested partly by 

 the excretions of glandular cells of the endosperm, and of the 

 embryo. The translocation of stored food from seeds, bulbs 

 tubers, etc., affords the most interesting examples of digestion in 

 which but little investigation has been carried out. 



339. Digestion of Starch. Two kinds of diastase have been 

 isolated that act hydrolytically upon starch. One is known as 

 the diastase of translocation, and is found in germinating seeds 



